State of the art resonant converters operate at switching frequencies just above the audible range, thus at 18 kHz or slightly higher. Modern resonant converters are operating at higher switching frequencies resulting in proportionally increased switching losses, if no means is applied to reduce them. Any switching losses incurred by associated power switches in a single switching cycle have to be reduced in order to limit the overall power losses. One method to reduce such switching losses is Zero Current Switching (ZCS), which is a soft switching method. Here switching, which means turning switches on or off, is only performed when at or near a zero crossing of a resonant current of the resonant converter occurs.
The ZCS method is common practice in soft switching converters, but is disadvantageous, since it inhibits the controllability of the output power of the resonant converter. A continuous control for all points of operation in a non-ZCS method in the common way does guarantee results in switching losses. In order to overcome this disadvantages a further switching method based on the ZCS method was provided by WO 2006/114719A1. There the conflicting requirements of good controllability on the one hand and reduction of switching losses by applying ZCS on the other hand is overcome. The suggested resonant DC/DC power converter in WO 2006/114719A1 has less power losses although a soft switching control mode is used. This is achieved by switching the power switches only at or near, which includes shortly before or shortly after, the zero crossing of the resonant current. Under this condition the switching time instances and the frequency of the applied converter voltage are self-adjusting and cannot be used to adjust the output power. The power is controlled instead by selecting a switch configuration, defined by which of the power switches of the converter are turned on or off, resulting in a converter that has an output power controllable over a wide range able to efficiently limit switching losses by ensuring ZCS for every switching cycle.
As a result, in WO 2006/114719A1 a control method has been described that allows zero current switching for all operating points while maintaining full controllability of the output voltage. Due to the zero current switching the power losses are very low. The method is based on the transfer of discrete power portions from the mains supply to the output of a power converter. The discrete power portions are generated by applying a voltage to the resonant circuit that is in-phase to the current (“+state”), a zero voltage (“0state”) or an anti-phase voltage (“−state”). The discretization of the applied power portions is given by the number of available control levels. For the three-level control method thus three different power levels plus (+), zero (0), minus (−) are valid (see also FIGS. 2 to 4, which will be described in more detail below).
Generally the required power for a specific operation point lies in between two power levels (e.g. between the plus power level and the zero power level). The outcome of the described control strategy is that for a certain amount of time the upper power level is activated and for another amount of time the lower power level is used. Nevertheless the average power in time should be identical to the required power. The result of applying the two different power levels is a variation of the output voltage from the required voltage with a certain frequency and amplitude depending on the operation point. This phenomenon is called chattering.
For operating points that are near the plus-level (see also FIG. 6, which will be described in detail below) there is not much surplus power available to increase the output after a dropdown of the voltage. At the same time the output voltage decreases rapidly if a zero-level is applied. As a consequence the chattering frequency of the output voltage becomes very low and the amplitude very large.
A self-evident recipe to avoid this large chattering is to forbid operating points near the plus-level power. This would require an over dimensioning of the high voltage generator leading to higher costs. Furthermore, it is not excluded by itself, that there are other intermediate operating points, which have a similar property. It is therefore an object of the invention to reduce the chattering effects during the operation of a resonant power converter. Thus, this invention proposes an improvement to reduce the voltage chattering without over dimensioning of the generator. This provides also an improvement of a common three-level control strategy.